This invention relates generally to burner assemblies in glass-ceramic cooktop appliances and more particularly to optical sensors having an increased field of view for such burner assemblies.
The use of glass-ceramic plates as the cooking surface in cooking appliances such as cooktops and ranges is well known. Such cooking appliances (referred to herein as glass-ceramic cooktop appliances) typically include a number of heating elements or energy sources mounted under the glass-ceramic plate, one or more sensors for measuring the glass-ceramic temperature, and an electronic controller. The glass-ceramic plate presents a pleasing appearance and is easily cleaned in that its smooth, continuous surface lacks seams or recesses in which debris can accumulate. The glass-ceramic plate also prevents spillovers from falling onto the energy sources below. The controller controls the power applied to the energy sources in response to user input and input from the temperature sensors.
In one known type of glass-ceramic cooktop appliance, the glass-ceramic plate is heated by radiation from one or more of the energy sources disposed beneath the plate. The glass-ceramic plate is sufficiently heated by the energy source to heat utensils placed on it primarily by conduction from the heated glass-ceramic plate to the utensil. Another type of glass-ceramic cooktop appliance uses an energy source that radiates substantially in the infrared region in combination with a glass-ceramic plate that is substantially transparent to such radiation. In these appliances, a utensil placed on the cooking surface is heated partially by radiation transmitted directly from the energy source to the utensil, rather than by conduction from the glass-ceramic plate. Such radiant glass-ceramic cooktop appliances are more thermally efficient than other glass-ceramic cooktop appliances and have the further advantage of responding more quickly to changes in the power level applied to the energy source. Yet another type of glass-ceramic cooktop appliance inductively heats utensils placed on the cooking surface. In this case, the energy source is an RF generator that emits RF energy when activated. The utensil, which comprises an appropriate material, absorbs the RF energy and is thus heated.
In each type of glass-ceramic cooktop appliance, provision must be made to avoid overheating the glass-ceramic plate. For most glass-ceramic materials, the operating temperature should not exceed 600-700.degree. C. for any prolonged period. Under normal operating conditions, the temperature of the glass-ceramic plate will generally remain below this limit. However, conditions can occur that can cause this temperature limit to be exceeded. Commonly occurring examples include operating the appliance with a small load or no load (i.e., no utensil) on the cooking surface, using badly warped utensils that make uneven contact with the cooking surface, and operating the appliance with a shiny and/or empty utensil.
To protect the glass-ceramic plate from extreme temperatures, glass-ceramic cooktop appliances ordinarily have some sort of temperature sensor for monitoring the temperature of the glass-ceramic plate. If the glass-ceramic plate approaches its maximum temperature, the power supplied to the energy source is reduced to prevent overheating. In addition to providing thermal protection, such temperature sensors can be used to provide temperature-based control of the cooking surface and to provide a hot surface indication, such as a warning light, after a burner has been turned off.
One known approach to sensing temperature in glass-ceramic cooktop appliances is to place a temperature sensor directly on the underside of the glass-ceramic plate. With this approach, however, the temperature sensor is subject to the high burner temperatures and is thus more susceptible to failure. Moreover, direct contact sensors are limited in the area of the glass-ceramic plate that they can monitor and can fail to detect hot spots that may form on the glass-ceramic plate. Thus, it is desirable to use an optical sensor that "looks" at the glass-ceramic plate from a remote location to detect its temperature.
For cost and mechanical reasons, it is advantageous to locate the optical temperature sensor concentric to and beneath the burner. In this location, however, the sensor will only sense a small region of the glass-ceramic plate that is directly above the center of the burner because of its relatively small field of view (typically about 80 degrees in conventional sensors). This means that a significant portion of the heated glass-ceramic would not be under the thermal protection afforded by the optical temperature sensing system. Furthermore, such optical sensors are susceptible to accumulations of dust that is released from the burner insulation during shipment or installation of the appliance. Such dust accumulations on the optical sensor can reduce its efficiency and accuracy.
Accordingly, it would be desirable to have an optical temperature sensor for glass-ceramic cooktop appliances that has a wide field of view and is less susceptible to dust accumulation than existing devices.